feat: Mock SystemTime::now() for the tests

Add a new crate `deltachat_time` with a fake `struct SystemTimeTools` for mocking
`SystemTime::now()` for test purposes. One still needs to use `std::time::SystemTime` as a struct
representing a system time. I think such a minimalistic approach is ok -- even if somebody uses the
original `SystemTime::now()` instead of the mock by mistake, that could break only tests but not the
program itself. The worst thing that can happen is that tests using `SystemTime::shift()` and
checking messages timestamps f.e. wouldn't catch the corresponding bugs, but now we don't have such
tests at all which is much worse.

src/sync.rs

@@ -319,7 +319,7 @@ impl Context {
 
 #[cfg(test)]
 mod tests {
-    use std::time::{Duration, SystemTime};
+    use std::time::Duration;
 
     use anyhow::bail;
 
@@ -329,6 +329,7 @@ mod tests {
     use crate::contact::{Contact, Origin};
     use crate::test_utils::TestContext;
     use crate::token::Namespace;
+    use crate::tools::SystemTime;
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_config_sync_msgs() -> Result<()> {